CINCINNATI (AFP) - White House hopefuls hit the ground on Monday (March 14) in a last-minute push for votes on the eve of a crucial new election test, in a toxic climate following an eruption of violence around Mr Donald Trump's race for the Republican nomination.

Dubbed "Super Tuesday 2" by US media, the latest major date in the run-up to November's presidential election will see Democratic and Republican primary contests in the states of Florida, Ohio, Illinois, Missouri and North Carolina.

The billionaire Trump is looking to build an insurmountable lead, but Republican rivals Texas Senator Ted Cruz, Florida Senator Marco Rubio and Ohio Governor John Kasich are more determined than ever to bar his path.

Mr Trump has jetted across the delegate-rich states in recent days, staring down criticism over clashes at a planned rally on Friday in Chicago - which many saw as a natural consequence of the violent tone of his campaign.

The Republican front runner's invective has targeted immigrants, Muslims, Hispanics and other minorities, journalists and the disabled - often to raucous approval from thousands of chanting partisans.

But as with each new controversy swirling around him, Mr Trump seemed unscathed by the uproar with polls suggesting he remained on a glide path toward the party nomination heading into Tuesday's make-or-break round of voting.

Mr Trump, 69, has rejected out of hand any suggestion that his rhetorical excesses have created a climate of violence, blaming supporters of Bernie Sanders for sowing trouble - and threatening to respond in kind by sending supporters to picket the Democratic candidate's rallies.

The 74-year-old senator from Vermont, who has pointedly refrained from personal attacks in the campaign, gave a furious retort at a CNN Democratic townhall event on Sunday, stating simply: "Donald Trump is a pathological liar."

"We have never, our campaign does not believe in and never will encourage anybody to disrupt anything," Mr Sanders added.

"It is clear that Donald Trump is running a very cynical campaign pitting groups of Americans against one another. He is trafficking in hate and fear," Democratic candidate Hillary Clinton said.

Mr Trump's Republican rivals, eager to bring him down a notch but also seeming shaken by the weekend violence, have also pounced on the frontrunner.

Senator Rubio, who is trailing in third place and like Mr Kasich faces a do-or-die test in Tuesday's vote in his home state, called Mr Trump's language "dangerous."

"If we reach a point in this country where we can't have a debate about politics without it getting to levels of violence and anger," he told CNN, "we're going to lose our republic." Mr Kasich has accused him of creating a "toxic" atmosphere.

Friday's troubles in Chicago saw ardent Trump supporters and opponents come to blows, after dozens of campaign stops where he has encouraged the crowd to verbally and physically mistreat protesters.

An anti-Trump super-political action committee broadcast a television advertisement airing clips of the Republican saying "I'd like to punch him in the face," and similarly harsh statements against protesters.

Mr Trump appeared Sunday to condone one particularly striking act of violence, when a demonstrator was sucker-punched as he was led by police from a rally last week in North Carolina, by saying his staff would look into paying the belligerent supporter's legal bills.

Tensions were palpable at Mr Trump's campaign stops, with Secret Service and police ringing the candidate, after Friday's chaotic scenes and a brief security scare caused the next morning when a protester burst on stage at an event in Ohio.

Mr Trump went on to claim the protester was linked to the Islamic State in Iraq and Syria (ISIS) - an assertion that he refused to disown despite it becoming apparent it was based on a crude video hoax.

"What do I know about it?" he told NBC. "All I know is what's on the Internet."

The man in question, a 22-year-old anti-racism activist named Thomas DiMassimo, told CNN on Sunday he had been seeking to wrench the microphone from Trump - and had no that he had no desire to cause harm.

"I was thinking that Donald Trump is a bully, and he is nothing more than that," Mr DiMassimo said. "He's truly just a coward. And he's opportunistic and he's willing to destroy this country for power for himself."

In Cincinnati, Ohio on Sunday, hundreds lined up to see Mr Trump despite the drizzle - while demonstrators massed chanting "Build bridges, Not walls" and "No Trump no KKK, no fascist USA."

A protester holding a Sanders sign interrupted the candidate before being escorted out, to a loud cheer from the crowd.

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